Zeolites and zeolite-like materials are oxides of one or more elements selected from silicon, aluminum, phosphorus, and other metal atoms and contain pores and cavities having a size that may range from a few angstroms to about 2 nanometers. In this application, the terms micropore, microporous and all their derivatives refer collectively to pores having a diameter of less than 2 nanometers.
Zeolites and zeolite-like materials are characterized by their chemical composition (e.g., Si:Al atomic or molar ratios), as well as their crystal framework connectivity, conveniently described by a topological model. For a given chemical composition, an infinite number of theoretical structures is possible. Zeolites with over 130 different topologies have been synthesized, characterized and assigned a three letter code as mentioned in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001).
Zeolites and zeolite-type materials are widely used in separation processes (ion exchange, selective sorption). In their acid form, zeolites and zeolite-like materials are acid catalysts, due to the combination of their strong acidity and molecular size- and shape-selectivity. Such catalytic reactions normally take place in the pores and cavities of zeolites and zeolite-type materials but intra-particle diffusion limitations and pore blocking can prevent accessibility to a large number of catalytic sites. Also, it is often desirable to use a co-catalyst in order to tailor the catalytic activity of zeolites and zeolite-type materials to the particular use. The presence of the co-catalyst in close proximity of the catalytic sites is typically required for optimal performance. However, pore sizes of a few angstroms or even a few nanometers make it very difficult to synthesize materials where co-catalysts are in close proximity to the active sites of zeolites or zeolite-type materials.
One way to increase intra-particle diffusion is to reduce the size of the zeolite or zeolite-type crystals. Various methods have been described to make small crystal size molecular sieves (see, for example, International Patent Publication Nos. WO 00/06492, WO 00/06493, and WO 00/06494). However, the colloidal behavior of very small particles makes them difficult to recover and handle, especially on an industrial scale. Moreover, reducing crystal size does not address the issue that the microporous nature of zeolite or zeolite-like materials makes it difficult to introduce co-catalysts in close proximity to the catalytically active sites.
Another way to increase intra-particle diffusion is to increase the pore and cavity size of the molecular sieve. The synthesis of ordered mesoporous aluminosilicate catalysts; with pores of about 2 nanometers or larger, such as MCM-41, has been reported in the literature (see, for example, Kresge et al., Nature, 1992, 359, 710; Beck et al, J. Am. Chem. Soc. 1992, 114, 10834; D. Zhao et al., Science, Vol. 279, pp. 548-552, 1998). In the context of the present invention, “mesopores” refers to pores having a diameter of from 2 to 50 nanometers and “macropores” refers to pores having a diameter of greater than 50 nanometers.
However, while such mesoporous materials offer good diffusion properties, they frequently lack the strong acidity of their microporous analogues, and, of course, the desirable shape selectivity of microporous zeolite and zeolite-like materials is lost. Therefore, various strategies have been developed to modify the physical and chemical properties of mesoporous materials. For example, U.S. Pat. No. 5,145,816 discloses post-synthesis functionalization of MCM-41 type materials. In addition, it is known to encapsulate metal oxides in the mesopores of MCM-41 materials, see, for example, Dapurkar et al., Catalysis Today, 68 (2001), 63-68.
It is also known to introduce zeolite-type microporous structure in such mesoporous materials (see, for example, International Patent Publication Nos. WO 00/15551, WO 01/17901, WO 2004/026473, and WO 2004/052537). Another method for making crystalline materials with mesopores and zeolite-type micropores is to generate mesopores by steaming or acid-leaching microporous zeolites (see, for example, A. H. Janssen et al., J. Phys. Chem. B (2002) Vol. 106, pp. 11905-11909; S. van Donk et al. Catal. Rev. (2003) Vol. 45, p. 297). Yet another method has been described by growing aluminosilicate crystals in a mesoporous support (US Patent Application Publication No. 2005/0013773 and Y. Tao et al., J. Am. Chem. Soc. (2003), Vol. 125, pp. 6044-6045) or by growing zeolite crystals in the presence of nanoscale carbon black particles, followed by carbon combustion (see, for example, M. Hartmann, Angew. Chem. Int. Ed. (2004), Vol. 43, 5880-5882).
One of the drawbacks of the above-mentioned methods is that it is quite difficult to control simultaneously the micropore size/shape distribution and the mesopore size/shape distribution in the materials, to achieve optimal catalytic performance and intra-particle diffusion. For this reason, several research groups have been looking for other methods of making crystalline materials having mesopores of uniform size together with zeolite or zeolite-like micropores. One such method involves starting from very small zeolite nuclei (often referred to as “seeds”), having a size in the order of 1 nanometer and assembling them hierarchically around regularly sized and shaped templates, said templates having a size in the range of from about 2 nanometers to about 100 nanometers, preferably from about 2 nanometers to about 50 nanometers. For example, Y. Liu et al. (J. Am. Chem. Soc. (2000), Vol. 122, pp. 8791-8792) disclose the assembly of a hexagonal aluminosilicate structure from seeds that would normally nucleate the crystallization of faujasitic zeolite type Y. The seeds are heated in the presence of cetyltrimethylammonium bromide to form the mesoporous hexagonal structure.
International Patent Publication No. WO 2004/014798 discloses a method of producing a microporous and mesoporous aluminosilicate material having a silica to alumina molar ratio of 2 to 50, preferably 4 to 10, in which a precursor solution of a faujasitic zeolite containing tetramethylammonium cations is formed, the precursor solution is aged and is then mixed with a solution of a cationic surfactant, such as a cetyltrimethylammonium compound. The resultant gel is then crystallized at 20 to 200° C. for between 30 minutes and 7 days.
S. P. B. Kremer et al. (Ph. D. thesis, Katholieke Universiteit Leuven, February 2004; Adv. Funct. Mater. (2002) Vol. 12, No. 4, pp. 286-292; Adv. Mater. (2003), Vol. 15, No. 20, pp. 1705-1707) disclose assembling “nanoslabs” to form materials having regular sized mesopores and micropores, referred to as “zeotiles” or “zeogrids”. The nanoslabs used in this work are building blocks that would normally generate zeolites having the Silicalite-1 (MFI) framework type. The nanoslabs were formed by hydrolysis of TEOS in a concentrated aqueous TPAOH solution. The nanoslabs were assembled into zeotiles or zeogrids, by precipitation in the presence of cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DCTAB) or Pluronic P123 triblock copolymer under acidic conditions. After removal of CTAB, DCTAB, or Pluronic P123, an exceptionally open and ordered mesopore network, made of a zeolite-type material, was obtained.